There are many desirable operational features available from fluorescent lamps, in distinction to incandescent lamps. For example, fluorescent lamps typically use substantially less electrical power and produce equal or greater illumination from the same or less electrical power consumption.
One of the difficulties associated with fluorescent lamps is that they require exterior control equipment to provide reliable operation and to obtain a reasonable longevity of use. Ballasts are required to limit the current that flows in an arc between filament electrodes known as cathodes located at each end of the lamp. Starters control the voltage between the cathodes to generate a high voltage ignition pulse which ignites the medium between the cathodes into a conductive plasma. Once the medium is ignited, the lamp can remain lit by applying the typical power supply voltage between the cathodes to sustain the plasma.
The complexity of controlling the operation of the lamp can present difficult problems and contribute to the unreliable operation and the premature failure of the lamp. To start or ignite the lamp, the current and voltage applied to the cathodes are controlled to preheat the cathodes to a sufficient temperature before a high voltage ignition pulse is applied between the cathodes to ignite the medium into a conductive plasma. The current applied to preheat the cathodes causes a thermionic coating on the cathodes to emit a cloud of electrons. If the cathodes have not been sufficiently preheated before the high voltage ignition pulse is applied, the cloud of electrons will be insufficient to support the initial arc and the lamp will remain unlit despite the application of the high voltage starting pulse.
The thermionic coating on the cathodes is severely eroded when the high voltage ignition pulse is applied to insufficiently heated cathodes. After significant erosion, the thermionic coating becomes incapable of generating sufficient electrons for starting the lamp on a reliable basis. Thus, erosion of the thermionic cathode coating severely reduces the usable lifetime of the lamp. A major contributing factor to the excessive erosion of the lamp cathodes is repeated application of the high voltage ignition pulses during unsuccessful attempts to start the lamp when the cathodes have been insufficiently heated.
Proper operation of the lamp is further complicated by interruptions in the power supplied to the lamp. During ignition of the lamp, momentary power interruptions can cause the cathodes to cool and result in the high voltage ignition pulse failing to ignite the medium into the plasma, thereby eroding the cathodes. During operation of a lighted lamp, momentary interruptions in power can cause the lamp to become extinguished due to cooling of the cathodes.
In response to the lamp failing to start or becoming extinguished, some fluorescent lamp starters will immediately attempt to restart the lamp by heating the cathodes and generating high voltage ignition pulses. There are no restrictions on the number and frequency of attempts to restart the lamp. Consequently, frequent interruptions in the power supply voltage, or repeated unsuccessful attempts to light the lamp may result in overheating the ballast and premature failure of the lamp due to erosion of the thermionic coating on the cathodes.
It is with respect to these and other considerations that the present invention has evolved.